In the manufacture of useful materials of the kind indicated, the solid ingredients are added to the liquid solvents in the mixer through a loading hatch in the top of the otherwise closed vessel. The loading of the solids is typically performed manually by the emptying of sacks of the solid materials into the mixer through the hatch while the beater-agitators are churning the mix, the solid materials typically being gum rubbers, soluble resins, clays, pigments, or the like.
The loading process may occur over a period of time which can exceed one hour, during which period the hatch remains open while the attendant dumps sackload after sackload of material into the churning mixer. During this period, and throughout the mixing process, which may extend to several hours per batch, the mechanical energy applied to the contents creates shearing forces within the mix, raising the temperature of the mix to the point where solvents, normally liquid at room temperature, are evaporated off in surprisingly large quantity, raising the pressure in the head space above the contents and escaping as vapor from the mixing vessel in various ways.
All such vessels are, of course, vented, many through a conservation vent, i.e., a pressure relief valve, which, in better installations, is connected to a stack which discharges the vapor to the atmosphere outside the plant building. Leakage of vapor through imperfect seals around the hatch cover is also not uncommon, as evident from the presence of liquid solvent on the outside of the mixing vessel adjacent the hatch, as well as from the heavy odor of the solvent vapors which permeate the plant atmosphere.
As the escape of solvent vapors during manual loading is not only inevitable but concentrated, the attendant is usually provided with special breathing equipment to avoid prolonged exposure to hazardous solvent vapors, but the amount of solvent discharged to the plant atmosphere in vapor form through the open hatch is substantial and typically compensated by increasing the volume of air movement through the plant interior. As commercially viable solvents are usually also combustible, the danger of fire, not to say explosion, is constant, notwithstanding provisions for preventing static electrical discharges, e.g., grounding the attendant handling the solids loading, and increasing the ambient humidity.
The recovery as liquid of vapor in the head space of closed storage vessels for volatile liquids by the use of heat-exchanger condensers is known from U.S. Pat. Nos. 1,490,782 and 1,545,352, but these prior art devices and systems do not contemplate the prevention of the escape of solvent vapors through the open loading hatch of a mixing vessel.